The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine or a printer In particular, it relates to a heater used in the image forming apparatus.
In recent years, heating apparatuses based on a film heating system have been proposed, and some of them have been put to practical use (Japanese Laid-Open Patent Application Nos.: 313182/1988, 263679/1989, 157878/1990, 44075-44083/1992, and 204980-204984).
In these heating apparatuses, an object to be heated in pressed on a heating member, with interposition of a sheet of heat resistant film between the object and the heating member, and the heat from the heating member is applied to the object through the heat resistant film while moving the object and the heat resistant film relative to the heating member. Such heating apparatuses can be used as means for fixing an unfixed toner image to a sheet of recording material to produce a permanent toner image.
Also, they can be used in a wide range of heating apparatuses, for example, an apparatus for heating a sheet of recording material to improve its surface properties such as glossiness, an apparatus for temporarily fixing a toner image, or a means for simply heating a material in the form of a sheet.
In the case of a heating apparatus based on a film heating system, which uses a heater with low thermal capacity, and very thin film, the temperature of the heating member quickly increases, eliminating the need for supplying the heater with current while the apparatus is on standby. Therefore, even if a sheet of recording material as an object to be heated is fed without pre-activating the heater, the heater can reach a predetermined temperature, sufficiently before the sheet of recording material reaches a predetermined fixing temperature. Consequently, a heating apparatus based on a film heating system can save electrical power, can shorten waiting time (starts up quickly to afford an on-demand operation), and also can reduce the rate of increase in the internal temperature of the main assembly of an image forming apparatus or the like; it is very effective and offers many advantages.
FIG. 20(a) is an enlarged schematic cross-section of the essential portion of a typical heating apparatus (image fixing thermal apparatus) based on a film heating system, and FIG. 20(b) is a partially cutaway schematic plan of a heating member.
A reference numeral 10 designates a heating member, which comprises a heater substrate 2 (electrically insulative base member), a patterned heat generating resistor 3, a patterned electrically conductive material 4, a patterned electrically conductive material 4a, patterned power supply electrodes 5 and 6, a surface protection layer 7, a safety element 8, and the like. The heat generating resistor 3 is deposited in a predetermined pattern on one (front surface) of the surfaces of the heater substrate 2, and the surface protection layer 7 covers the patterned that generating resistor 3 and the patterned electrically conductive material 4. The safety element 8 comprises a thermal fuse, a thermal switch, and the like, and is placed in contact with the other surface (back surface) of the heater substrate 2.
The heater substrate 2 is a thin, rectangular member, measuring 240 mm in length, 10 mm in width, and 1 mm in thickness. Its longitudinal direction is perpendicular to a direction A in which a heat resistant film F and a recording material P as an object to be heated are conveyed. It is composed of ceramic material such as alumina or the like, being heat resistant, electrically insulative, and low in thermal capacity. The heat resistant film F will be described later.
The patterned heat generating member 3 is a narrow strip of electrically resistive material, which extends in the longitudinal direction of the ceramic substrate. It is created by coating paste of electrically resistive material such as silver/palladium (Ag/Pb) or Ta2N, on the ceramic substrate to a thickness of 10 xcexcm and a width of 1-3 mm, and then baking it. The coating method is a screen printing method or the like.
The patterned electrically conductive material 4 is a narrow strip of electrically conductive material, which extends on the ceramic substrate also in the longitudinal direction of the ceramic substrate, that is, substantially parallel to the patterned heat generating resistor 3.
Two patterned power supply electrodes 5 and 6 are disposed in parallel at one longitudinal end of the surface of the ceramic substrate 2.
One end of the patterned heat generating resistor 3 is connected to the patterned power supply electrodes 5 by the patterned conductive material 4a, and the other is connected to the patterned power supply electrode 6 by the patterned conductive material 4.
The patterned conductive material 4, the patterned conductive material 4a, the patterned power supply electrode 5, and the patterned power supply electrode 6 are all created by coating paste which contains electrically conductive material such as Ag, on the surface of the ceramic substrate 2, in a predetermined pattern, and baking it. The coating method is a screen printing method or the like.
The patterned heat generating resistor 3 and the patterned conductive material 4, which are substantially parallel to each other, constitute a current path through which electrical current flows in the longitudinal direction of the ceramic substrate 2 between the patterned power supply electrodes 5 and 6 disposed at one ed of the ceramic substrate 2.
The heating member 10 is fixed to a rigid and heat resistant heating member holder (stay) 11 in such a manner that the surface with the patterned heat generating member 3 is downwardly exposed.
An alphabetic reference F designates a sheet of 40-100 xcexcm thick heat resistant film such as polyimide film or the like, and a reference numeral 9 designates a pressing roller as a pressing member for pressing the film F on the surface protection layer 7 which constitutes the surface on which the film F slides.
The film F moves, being pressed on the heating member 10 by the pressing roller 9 and sliding on the surface of the heating member 10 (surface of the surface protection layer 7), in the direction indicated by an arrow mark, at a predetermined speed, as it is rotationally driven by the force from an unillustrated driving means, or by the pressing roller 9 which doubles as a driving means.
As electrical power is supplied to the patterned power supply electrodes 5 and 6 from an unillustrated power supply circuit, the patterned heat generating resistor 3 generates heat across its entire length, increasing the temperature of the heating member 10. The increased temperature of the heating member 10 is detected by an unillustrated temperature detecting element. The detected temperature is fed back to an unillustrated temperature control circuit to control the power supply to the patterned heat generating resistor 3, so that the temperature of the heating member 10 is kept at a predetermined level.
The heating member 10 and the pressing roller 9 forms a heating nip N (fixing nip or compressing nip), in which the film F is pinched. After the heating member 10 is heated to the predetermined temperature by supplying the patterned heat generating resistor 3 with power, and the film F is caused to slide on the heating member 10, the recording material P, that is, the object to be heated, on which a toner image is to be fixed, is introduced into the heating nip N, between the pressing roller 9, and the film F which is sliding on the heating member 10, so that the recording material P is conveyed through the heating nip N together with the film F. While the heating material P is passing through the heating nip N, the heat from the heating member 10 is transmitted to the recording material P through the heat resistant film F to thermally fix to the surface of recording material P, the unfixed visible image (toner image) carried on the recording material P After passing through the heating nip N, the recording material P is separated from the surface of the film F and is conveyed further.
The safety element 8 is disposed in contact with the back side of the heating member 10, that is, the back surface of the heater substrate 2, so that it is heated by the heat from the heating member 10. It plays a role in instantly cutting off the power supply to the heat generating resistor 3 so that the apparatus is prevented from smoking or catching fire due to the overheating of the heating member 10.
Thus, the thermal capacity of the safety element 8 disposed in contact with the back surface of the heating member 10 is rendered as small as possible. In spite of such a precaution, the thermal capacity of the safety element 8 sometimes becomes larger than that of the heating member holder 11. If this situation occurs, the temperature of the heating member 10, adjacent to the contact between the heating member 10 and the safety element 8, becomes lower than the temperature across the rest of the heating member 10. As a result, the fixing performance of the image heating apparatus is adversely affected. For example, an object to be heated is nonuniformly heated, or a time lag occurs as the safety element 8 responds to a heater control failure.
The primary object of the present invention is to provide a heater which is prevented by a temperature detecting element, from being nonuniformly heated and an image heating apparatus comprising such a heater.
Another object of the present invention is to provide a heater, on the substrate of which a plurality of resistive member and a plurality of temperature detecting elements are disposed, wherein a first portion of at least one resistive member, which corresponds to the location of the temperature detecting element, has a resistance value higher than that of a second portion of the same resistive member, and an image heating apparatus comprising such a heater.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.